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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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422                             * ALASKAN GLACIER STUDIES
than in the western zone, because it is revealed in more abundant sections and is slumping more continually, as is shown by the general absence of vegetation. The line between these two zones is a fairly sharp one in some places, with an east-facing bluff between the forested western zone of dense vegetation and the relatively-barren zone east of it. There are a very few scattered shrubs, mostly alders, which appear to be not over a year or two old, for the largest were only a foot or two in height, and the stems but a quarter of an inch to an inch in diameter, but those that we cut down had from 14 to 22 annual rings. The stunted condition may be due either (a) to the glacier wind and the coldness of the soil, or (b), as we consider more likely, to the shifting of the soil with slumping, as the ice underneath melts, killing or retarding the growth of the moss, grass, and shrubs. This process was actively in progress during our visit in 1910.
In this zone, especially near the western edge, there were large depressions in the ice, with a depth of from 80 to 100 feet, often containing ponds. At their edges the moraine, which was 2 to 3 feet thick, was continually sliding down into the water, and the ice thus revealed (PL CLXII) was black with debris, clearly indicating the source of the material making up the ablation moraine. There were also a few long, narrow valleys without lakes, one of them 150 feet or more in depth, with steep walls of bare ice, suggesting the enlargement of a great crevasse. None of these valleys was continuous for a great distance. Farther east there were many debris cones, rising 15 to 25 feet above the general level, some of them made up of'till with striated, subangular bowlders, and containing no ice. Many of these debris cones occur along lines, forming discontinuous ridges of distinctive rock, sometimes differing from the general rock material of the ablation moraine, sometimes composed of coarse stony material, rising slightly above the general level of an area of finer debris. These rows of de'bris cones are interpreted as the sites of former crevasses.
Three pronounced ridges close to the Copper River, near the southern border of the zone of thick ablation moraine (near Photo. Sta. M, Map 9), were from 10 to 15 feet high, about 100 feet apart, and made up of very coarse, angular rocks. There w^s slumping in one of these between 1900, when Schrader took a photograph, and our visit in 1910, showing that this area is still underlain by ice. Whether these three ridges are crevasse deposits, or recessional moraines is uncertain.
A narrow strip of moraine on the bank of Copper River and at the extreme southwestern point of the area of thickest ablation moraine, does not seem to be slumping and may contain no ice at present. There are large ponds in it and it apparently has Hot changed at all between 1884 and 1910.
The northern margin of the river and lake in the zone of thick ablation moraine is made up of (a) moderately steep cliffs of ice from which the debris is sliding, (b) partly of sand deposited by the Copper River and made into a broad sloping beach by the iceberg waves and (c), in the part bordering the channel by which the main river enters the lake, of very coarse stream-rounded bowlders.
Zone of Thin Ablation Moraine. In the third zone (Thin Ablation Moraine, Fig. 63), the ablation moraine forms an exceedingly attenuated veneer in most places, and bears no vegetation whatsoever (PL CLXIII). It is evidently an expanded extension of the northern lateral moraine of the valley glacier, and is separated from the barren zone of thick ablation moraine by a well-defined line, a low, west-facing cliff 5 or 10